Offset Printing Process by giuliacamedda

offset printing process

Giuliacamedda d a n i e l h i r c a m e l i n da h o rvat h pa u l a a r d e l e a n

co n t e n t s h i s t o r y p r i n t i n g p r o c e s s v a r i a t i o n s sheetfedoffset i n k s & c o bibliography& s i t o g r a p h

HISTORY

The first lithographic offset printing press was created in England around 1875 and was designed for printing on metal. The offset cylinder was covered with specially treated cardboard that transferred the printed image from the litho stone to the surface of the metal. About five years later, the cardboard covering of the offset cylinder was changed to rubber, which is still the most commonly used material. The first person to use an offset press to print on paper was most likely American Ira Washington Rubel in 1903. He got the idea accidentally by noticing that whenever a sheet of

paper was not fed into his lithographic press during operation, the stone printed its image to the rubber-covered impression cylinder, and the next impression had an image on both sides: direct litho on the front and an image from the rubber blanket on the back. Rubel then noticed that the image on the back of the sheet was much sharper and clearer than the direct litho image because the soft rubber was able to press the image onto the paper better than the hard stone. He soon decided to build a press which printed every image from the plate to the blanket and then to the paper.

Brothers Charles and Albert Harris independently observed this process at about the same time and developed an offset press for the Harris Automatic Press Company soon after. Harris designed his offset press around a rotary letterpress machine. It used a metal plate bent around a cylinder at the top of the machine that pressed against ink and water rollers. A blanket cylinder was positioned directly below, and in contact with, the plate cylinder. The impression cylinder below pressed the paper to the blanket in order to transfer the image to the sheet (see diagram). While this ba-

sic process is still used today, refinements include two-sided printing and web feeding (using rolls of paper rather than sheets). By 1914, Kawamura Kijuro Shoten saw that the offset press was gaining popularity in Japan, and the company wanted in on the innovations being created with regard to the press. The Kawamura Kijuro Shoten company began experimenting to create the best pigment ink for use with the printing press. By the time they released their product in 1915, there were more than 100 presses within Japan. Sales for the company skyrocketed. Artists began to use offset prin-

-ing to reproduce their works of art. They found that this printing method produced a superior and sharper image than traditional letterpress printing was able to produce. At this time, illustrations became much more popular in books and for prints since the method of reproducing illustrations became much more simple and cost-effective. During the 1950s , offset printing became the most popular form of commercial printing improvements were made in plates, inks and paper, maximizing the techniqueâ&#x20AC;&#x2122;s superior production speed and plate durability. Today, the majority of printing, including newspapers, is done by the offset process,

although digital printing has greatly increased in popularity due to demand and cost advantages for low quantity runs. In the 1960s, offset printing overtook letterpress printing in terms of popularity. During this time, methods were developed that allowed presses to electronically set their type and the images they wanted printed on the press. This began the process of eliminating the need for a metal plate with recesses - the creation of which would often expose press employees to dangerous fumes. This trend continued into the 1970s when computers with lasers were developed to create the image that would be printed.

CHRONOLOGICAL H I STO RYO F P R I N T I N G

Woodblock printing 200 CE Movable type 1040 Printing press 1377 Etching c. 1515 Mezzotint 1642 Aquatint 1772 Lithography 1796 Chromolithography 1837 Rotary press 1843 Hectograph 1869

Offset printing

1875

Hot metal typesetting 1884 Mimeograph 1886 Photostat and Rectigraph 1907 Screen printing 1910 Spirit duplicator 1923 Xerography 1938 Phototypesetting 1949 Inkjet printing 1951 Dye-sublimation 1957 Dot matrix printing 1968 Laser printing 1969 Thermal printing c. 1972 Digital press 1993

printing process

The most common kind of offset printing is derived from the photo offset process, which involves using light-sensitive chemicals and photographic techniques to transfer images and type from original materials to printing plates. In current use, original materials may be an actual photographic print and typeset text. However, it is more common â&#x20AC;&#x201D; with the prevalence of computers and digital images â&#x20AC;&#x201D; that the source material exists only as data in a digital publishing system. Offset Lithographic printing on to a web (reel) of paper is commonly used for printing

of newspapers and magazines for high speed production. In this process, ink is transferred from the ink duct to the paper in several steps:

The ink duct roller delivers ink from the ink duct to the ink pyramid, also called the Ink Train.

The ductor roller, sometimes called a vibrator roller due to its rapid back and forth motion, transfers ink from the duct roller to the first distribution roller. It is never in contact with both rollers at the same time.

The distribution rollers evenly distribute the ink. The first distribution roller picks up the ink from driving rollers, and the last distribution rollers transfer the ink to the form rollers.

Ink form rollers transfer ink from the last distribution rollers on to the printing plate.

The printing plate transfers the ink to the offset cylinder (typically called the blanket cylinder) usually covered with a rubber â&#x20AC;&#x153;blanket.â&#x20AC;?

The paper is then pressed against the blanket cylinder by the impression cylinder, transferring the ink onto the paper to form the printed image.

The transfer rollers transfer ink between the ink-absorbing and ink-delivering driving rollers. Driving rollers roll against the distribution rollers and either absorb or deliver ink, depending on their placement.

water rollers

ink rollers

plate cylinder

offset cylinder

paper

impression cylinder

The actual process of printing is quite involved. One of the most important functions in the process is pre-press production. This stage makes sure that all files are correctly processed in preparation for printing. This includes converting to the proper CMYK color model, finalizing the files, and creating plates for each color of the job to be run on the press. Every printing technology has its own identifying marks, as do offset printing. In text reproduction, the type edges are sharp and have clear outlines. The paper surrounding the ink dots is usually unprinted. The halftone dots are always hexagonal though there are different screening methods.

VARIATIONS

Blanket-to-blanket: A printing method in which there are two blanket cylinders through which a sheet of paper is passed and printed on both sides Blanket-to-blanket presses are considered a perfecting press because they print on both sides of the sheet at the same time. Since the b-to-b press has two blanket cylinders, making it possible to print on both sides of a sheet, there is no impression cylinder. The opposite blanket cylinders act as an impression cylinder to each other when print production occurs. This method is most utilized on offset presses designed for envelope printing.

Blanket-to-steel: A printing method similar to a sheet offset press; except that the plate and cylinder pressures are quite precise. Actual squeeze between plate and blanket cylinder is optimal at .005â&#x20AC;?; as is the squeeze or pressure between the blanket cylinder and the substrate. Blanket-to-steel presses are considered one-color presses. In order to print the reverse side, the web is turned over between printing units by means of turning bars. The method can be used to print business forms, computer letters, and direct mail advertising. Dry Offset Printing:

Dry Offset Printing: A printing process which uses a metal backed photopolymer relief plate, similar to a letterpress plate, but, unlike letterpress printing where the ink is transferred directly from the plate to the substrate, in dry offset printing the ink is transferred to a rubber blanket before being transferred to the substrate. This method is used for printing on injection molded rigid plastic buckets, tubs, cups and flowerpots. Variable-size printing: Uses removable printing units, for one-sided and blanketto-blanket two-sided printing.

Keyless offset: A printing process that is based on the concept of using fresh ink for each revolution by removing residual inks on the inking drum after each revolution. It is suitable for printing newspapers.

sheet-fed offset

Sheet-fed refers to individual sheets of paper or rolls being fed into a press via a suction bar that lifts and drops each sheet onto place. A lithographic (“litho” for short) press uses principles of lithography to apply ink to a printing plate, as explained previously. Sheet-fed litho is commonly used for printing of short-run magazines, brochures, letter headings, and general commercial (jobbing) printing. In sheet-fed offset, “the printing is carried out on single sheets of paper as they are fed to the press one at a time.” Sheet-fed presses use mechanical registration to relate each sheet to one another to ensure that they are reproduced with the same imagery in thesame position

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Perfecting press: A perfecting press, also known as a duplex press, is one that can print on both sides of the paper at the same time. Web and sheet-fed offset presses are similar in that many of them can also print on both sides of the paper in one pass, making it easier and faster to print duplex. Offset duplicators: Small offset lithographic presses that are used for fast, good quality reproduction of one- and two-color copies in sizes up to 12” by 18”. Popular models

were made by A.B. Dick, Multilith, and the Chief and Davidson lines made by A.T.F./Davidson. Offset duplicators are made for fast and quick printing jobs; printing up to 12,000 impressions per hour. They are able to print business forms, letterheads, labels, bulletins, postcards, envelopes, folders, reports, and sales literature. Feeder system: is responsible for making sure paper runs through the press correctly. This is where the substrate is loaded and then the system is correctly set up to the certain specifications of the substrate to the press.

Printing system: The Printing Unit consists of many different systems. The dampening system is used to apply dampening solution to the plates with water rollers. The inking system uses rollers to deliver ink to the plate and blanket cylinders to be transferred to the substrate. The plate cylinder contain all of the imaging are mounted. Finally the blanket and impression cylinders are used to transfer the image to the substrate running through the press. Delivery system: is the final destination in the printing process while the

paper runs through the press. Once the paper reaches delivery, it is stacked for the ink to cure in a proper manner. This is the step in which sheets are inspected to make sure they have proper ink density and registration. Slur: Production or impact of double image in printing is known as ‘slur’. Web-fed: refers to the use of rolls (“webs”) of paper supplied to the printing press. Offset web printing is generally used for runs in excess of 5-10.000 impressions. Ex. of web printing are

magazine, magazine ads, newspapers, direct mail, catalogs, and books... Web-fed presses are divided into two general classes: “cold” or “non-heatset,” and “heatset” offset web presses; the difference being how the inks that are used dry. Cold web offset printing dries through absorption into the paper, while heatset utilizes drying lamps or heaters to cure or “set” the inks. Heatset presses can print on both coated (slick) and uncoated papers, while coldset presses are restricted to uncoated paper stock, such as newsprint. Some coldset web presses can be fitted with heat dryers, or ultraviolet lamps (for use with UV-curing inks). It is also possible to add a drier to a cold-set press.

This can enable a newspaper press to print color pages heatset and black and white pages coldset. Web offset presses are beneficial in long run printing jobs, typically press runs that exceed ten or twenty thousand impressions. Speed is a determining factor when considering the completion time for press production; some web presses print at speeds of 3,000 feet per minute or faster. In addition to the benefits of speed and quick completion, some web presses have the inline ability to cut, perforate, and fold. Heatset web offset: This fset

subset of web printing uses

ofinks

which dry by evaporation in a dryer typically positioned just after the printing units. This is typically done on coated papers, where the ink stays largely on the surface, and gives a glossy high contrast print image after the drying. As the paper leaves the dryer too hot for the folding and cutting that are typically downstream procedures, a set of â&#x20AC;&#x153;chill rollsâ&#x20AC;? positioned after the dryer lowers the paper temperature and sets the ink. The speed at which the ink dries is a function of dryer temperature and length of time the paper is exposed to this temperature. This type of printing is typically used for magazines, catalogs, inserts and other medium-to-high volume, quality production runs.

Coldset web offset: This is also a subset of web offset printing, typically used for lower quality print output. It is typical of newspaper production. In this process, the ink dries by absorption into the underlying paper. A typical coldset configuration is often a series of vertically arranged print units and peripherals. As newspapers seek new markets, which often imply higher quality (more gloss, more contrast), they may add a heatset tower (with a dryer) or use UV (ultraviolet) based inks which â&#x20AC;&#x153;cureâ&#x20AC;? on the surface by polymerisation rather than by evaporation or absorption.

Web-fed vs. sheet-fed Sheet-fed presses offer several advantages. Because individual sheets are fed through, a large number of sheet sizes and format sizes can be run through the same press. In addition, waste sheets can be used for make-ready (which is the testing process to ensure a quality print run). This allows for lower cost preparation so that good paper is not wasted while setting up the press, for plates and inks. Waste sheets do bring some disadvantages as often there are dust and offset powder particles that transfer on to the blankets and plate cylinders, creating

imperfections on the printed sheet. This method produces the highest quality images. Web-fed presses, on the other hand, are much faster than sheet-fed presses, with speeds in excess of 20,000 cut-offs per hour. (Cut-off is the paper that has been cut off a reel or web on the press. The length of each sheet is equal to the cylinderâ&#x20AC;&#x2122;s circumference.) The speed of web-fed presses makes them ideal for large runs such as newspapers, magazines, and comic books. However, webfed presses have a fixed cut-off, unlike rotogravure or flexographic presses, which are variable.

inks

Offset printing inks are compounded especially for use on offset presses. They must be able to withstand reaction with the press fountain solution it encounters on the dampened offset plate. Ideally, the ink on the ink roller should not absorb water (water-in-ink emulsification), nor should the ink break down and combine with the fountain solution on the non-printing areas of the plate (ink-in-water emulsification). Either of these emulsification problems will tend to impair the body, color, or drying qualities of the ink, or cause tinting on the non-printing areas of the plate and printed sheets.

Composition of offset ink Ink is made of three main ingredients: Pigment, which is the coloring material in the ink; Vehicle, which is the liquid that holds the particles of pigment; andModifiers, which control the drying of the ink as well as other factors such as smell, scuff resistance, and fading. Pigment: There are two basic types of pigment used in offset printing inks. Organic pigment, which is made from carbon, is used for making black ink. Inorganic pigments,

which are made by mixing various chemicals together, are used for colored inks. For example, sulfur, silica, or china clay can be combined with either soda ash or sulfate salts to make ultramarine blue ink. Vehicle: is the liquid that holds the particles of pigment and carries them to the paper. There are two kinds of vehicles used in offset inks: oils such as soya oil or linseed oil (which is a yellowish oil made from flax); and synthetic vehicles, which are liquids resulting from the mixture of chemicals.

For example, phenol and formaldehyde mixed together make phenolic resins, sometimes used in printing inks as a vehicle. MODIFIERS: Modifiers are ingredients added to the ink to control dryingand other qualities such as smell and resistance to fading.

VISUAL PROPERTIES Visual properties of inks are a function of the colorant or pigment, in relation to the vehicle system used. They include color, transparency or opacity, and gloss.

Opacity must to be suited in the use of the ink.Itâ&#x20AC;&#x2122;s tested by spreading a sample of ink with an ink knife over a wide black line printed on a sheet of paper. The amount of covering is then compared to a standard to determine if the opacity is correct. INK TRASPARENCY:

INK OPACITY: Ink opacityis the ability to hide the color beneath it. Sometimes, an ink with little opacity is needed, such as when overlapping two colors to create a third color. Other times, very opaque ink is needed to completely cover any color under it.

Ink transparency refers to the opposite of opaque. A transparent ink does not hide the color beneath it, but mixes with it to create a third color. All inks used to print full color work must be transparent. The choice of colorant and the degree to

which it is dispersed through the vehicle are the most important factors in determining the transparency or opacity of an ink.

GLOSS: refers to an inkâ&#x20AC;&#x2122;s own ability to reflect light, and depends upon the lay or smoothness of the ink film on the substrate surface. Generally, the higher the ratio of vehicle to colorant, the smoother the lay, and the higher the gloss. Application of a thicker ink film tends to improve gloss while penetration into the substrate tends to reduce.

bibliography sitography

BIBLIOGRAPHY: -Ambrose G., Harris P., Il manuale del graphic design. Progettazione e produzione, Roma, Zanichelli, 2009. -Pipes A., Production For Graphic Designers., London, Lawrence King, 2004. SITOGRAPHY: http://www.wikipedia.com http://www.griso-chemie.com http://www.weboffsetprint.com http://www.offsetprintingtechnology.com http://www.sitographics.it